Substrate glass for optical interference filters with minimal wave length shift

ABSTRACT

A silicate based composition for optical glass used as a substrate for thin film optical interference filters having a stable transmission band center wavelength and bandwidth has a relatively high coefficient of thermal expansion, high Young&#39;s modulus and high optical transmittance in the near infrared (NIR) wavelength range of about 950 nm to about 1600 nm. The coefficient of thermal expansion of the glass composition is adjustable to particular values to result in minimal wavelength shift in filters made by depositing thin films of particular dielectric materials onto a substrate made of the glass, the composition being varied from a preferred baseline composition consisting of about 43.2% SiO2, 7% Al2O3, 12.7% CaO, 7.3% SrO, 7.8% Li2O, 13.2% Na2O, 8.0% K2O, 0.7% ZrO, and 0.1% Sb2O3, the baseline composition having a coefficient of thermal expansion of about 112×10 −7 /° C. over the temperature range of −30° C. to +70° C., a Young&#39;s modulus E of 88.3 Gpa, and an optical transmittance of 90.9% at 1550 nm for an 8 mm thick sample plate.

BACKGROUND OF THE INVENTION

[0001] A. Field of the Invention

[0002] The present invention relates to glasses suitable for beingshaped into small, flat windows used as substrates onto which thin filmcoatings of a suitable material are applied to form optical interferencefilters. More particularly, the invention relates to a glass compositionin which relative proportions of components thereof may be varied toadjust the thermal expansion coefficient of the glass to relatively highvalues which are tailored to suit characteristics of particular thinfilm coatings, enabling fabrication of optical interference filtershaving transmission bands which shift minimally in wavelength inresponse to varying temperatures and other environmental stresses.

[0003] B. Description of Background Art

[0004] Optical interference filters fabricated by applying onto a thin,flat transparent glass substrate one or more coatings of dielectric orconductive films each having a thickness which is a multiple ofone-quarter wavelength of a light of a particular wavelength are wellknown and widely used. Optical thin film interference filters are ofvarious types, including low pass or high pass filters which transmitlight having wavelengths longer or shorter than a particular cut-offwavelength, band pass filters which transmit only light havingwavelengths within a particular band of wavelengths, and notch filterswhich transmit light over a range of wavelengths comprising a pass bandwhile reflecting or absorbing light in a smaller range of wavelengthscentered about a notch wavelength contained within the pass band. Onecommon example of a thin film optical interference filter is the opticalanti-reflection coating on the lenses of binoculars, which formstherewith a relatively wide band pass filter for light within thevisible spectrum, minimizing reflections from the surface of objectivelenses of light received from an object viewed and maximizingtransmission of light through the eyepieces to the eyes of a viewer.

[0005] Performance requirements for band pass filters used in certainoptical communications applications, such as Dense Wavelength DivisionMultiplexing (DWDM) are more demanding than those imposed on otherapplications for optical interference filters, as will now be explained.

[0006] In Dense Wavelength Division Multiplexing (DWDM) light energy indiscrete wavelength bands is modulated with radio frequency signalscontaining audio, video or digital information including telephoneconversations, television transmissions and digital computer datasignals, in a typical DWDM system, an optical signal generated by alaser and having a center wavelength in the near infrared portion of theelectromagnetic spectrum, e.g., 1.5 microns, (1500 nanometers) issubdivided into a plurality of wavelength bands which comprise separateoptical carrier channels. Each optical carrier channel may have abandwidth of about 0.2 nanometers, and be separated from one another byabout 8 nanometers. The amplitude or phase of each of the opticalcarrier channel signals is modulated by a plurality of radio frequencysub-carrier channel signals, e.g., having a bandwidth of about 25 GHzand a channel separation of about 100 GHz. Each RF channel is in turnmodulated at a lower frequency with information such as digitizedtelephone conversation signals, television signals or other digitaldata.

[0007] In a simplified example, a DWDM system may employ separateoptical carrier channel signals each having a band width of 0.2nanometer and center wavelengths of 1500, 1499.2, 1498.4, 1500.8, and1501.6, nanometers, respectively. The plurality of optical carrierchannel signals is optically combined or “multiplexed” onto a singleoptical beam, which may then be transmitted on a single optical fiber.Optical multiplexing may be performed using a resonant cavity filter,such as the one depicted in FIG. 3 of U.S. Pat. No. 5,953,134, theentire specification of which is incorporated herein by reference. Theresonant cavity filter described therein employs a plurality ofindividual interference filters, each one being highly transmissive tolight in a particular wavelength band, and highly reflective to allother wavelengths of light.

[0008] At the receiving end of an optical fiber or other transmissionmedia through which combined or multiplexed optical signals aretransmitted, a resonant optical cavity provided with a separateinterference filter for each optical channel may be used to divide or“de-multiplex” the combined signal into separate optical beams which arearranged to impinge on a plurality of separate photo-detectors, one foreach optical channel, thus allowing signal information contained on eachoptical carrier channel to be directed to appropriate destinations forthe information signals on each channel, where the information may berecovered by demodulating the optical carrier signal.

[0009] Because of the very narrow bandwidth and close center wavelengthspacing required of interference filters used for DWDM applications asdescribed above, both the center wavelength and bandwidth must remainprecisely fixed in spite of variations in ambient temperature, humidity,and other environmental conditions encountered by DWDM systems.Otherwise, data transmitted over adjacent optical channels couldintermix, be reduced substantially in signal-to-noise ratio, or be lostentirely. Thus, the glass which is used for substrates onto whichdielectric coatings are applied to form interference filters for use inDWDM applications must have properties which differ from those ofexisting glass compositions, for the following reasons.

[0010] Conventional glasses may be broadly categorized as “soft” or“hard.” Soft glasses typically have a linear coefficient of thermalexpansion ( or “CTE”) of greater than 60×10⁻⁷, while hard glassesusually have a CTE of less than 60×10⁻⁷. The softer glasses generallyhave a lower Young's modulus and are generally more subject to surfacedegradations by environmental conditions such as high temperatures,humidity and/or corrosive atmospheres. On the other hand, it has beendetermined that glass used as a substrate for receiving dielectriccoatings to form highly wavelength-stable interference filters of thetype required for DWDM applications should have a relatively highYoung's modulus, to provide required dimensional stability, but mustalso have a thermal coefficient of expansion which is substantiallylarger than that typical of hard glasses. Moreover, it has been foundthat to maintain the center wavelength and bandwidth of opticalinterference filters stable enough for use in DWDM applications, thethermal expansion coefficient of the glass filter substrate must berather precisely tailored to suit properties of the particulardielectric coatings applied to the substrate. Typical dielectric coatingmaterials include oxides of titanium, tantalum, niobium, silicon andaluminum, and other substances. It is believed that better wavelengthstability is obtained using glass substrates with higher coefficients ofexpansion, because the high CTE's tend to produce compressive stressesin metal oxide coatings deposited on the glass, when the glass coolsdown to ambient temperature after being heated to a temperaturetypically exceeding 200° C. during the coating process, which istypically done in a low pressure chamber.

[0011] In apparent recognition of the desirability of providing a glasswith special properties for use as substrates for DWDM interferefilters, NA0YURI, in Patent Publication Number EP 1081512 disclosed aglass for a light filter stated to be capable of preventing variationsof refractive index in a band pass filter, to have a coefficient ofthermal expansion within a range from 90×10⁻⁷/° C. to 120×10⁻⁷/° C.within a temperature range of −20° C. to +70° C., and, preferably aYoung's modulus of 75 GPa or over, a Vickers hardness of 550 or over,and light transmittance for plate thickness of 10 mm of 90% or overwithin a wavelength range of 950 nm to 1600 nm.

[0012] The present invention was conceived of to provide an improvedglass composition which is particularly well suited for use as asubstrate for optical interference filters, of the type used in densewave division multiplexing (DWDM)

OBJECTS OF THE INVENTION

[0013] An object of the present invention is to provide a glasscomposition which has a relatively high thermal coefficient ofexpansion.

[0014] Another object of the invention is to provide a glass compositionwhich has a relatively high thermal coefficient of expansion, and arelatively high Young's modulus.

[0015] Another object of the invention is to provide a glass compositionwhich has a relatively high coefficient of thermal expansion, arelatively high Young's modulus, and a relatively high lighttransmittance.

[0016] Another object of the invention is to provide a glass compositionwhich is suitable for use as a substrate to receive thin film coatingswhich form in combination with the substrate an interference filterhaving a transmission band which is stably positioned at a selectedwavelength region.

[0017] Another object of the invention is to provide a glass compositionwhich has a relatively high thermal coefficient of expansion which maybe varied in a predetermined manner as a function of variations inpercentage composition of components of the glass.

[0018] Another object of the invention is to provide a glass compositionwhich is suitable for use as a substrate for optical interferencefilters having a transmission band which is stably centered on aparticular wavelength.

[0019] Another object of the invention is to provide a glass compositionwhich is suitable for use as a substrate for optical interferencefilters of the type used in Dense Wave Division Multiplexing (DWDM), thecomposition having a thermal coefficient of expansion adjustable byadjusting proportions of components of the composition to values in theapproximate range of 105 to 120×10⁻⁷/° C. over a temperature range ofapproximately −30° C. to +70° C., a Young's modulus of 85 Gpa orgreater, an optical transmittance of 91% or greater over a wavelengthrange from about 1300 nm to about 1600 nm, for a substrate having athickness of 8 mm or less, and environmental resistance to surfacedamage when exposed to high temperature, high humidity environments forextended periods of time.

[0020] Various other objects and advantages of the present invention,and its most novel features, will become apparent to those skilled inthe art by perusing the accompanying specification and claims.

[0021] It is to be understood that although the invention disclosedherein is fully capable of achieving the objects and providing theadvantages described, the characteristics of the invention describedherein are merely illustrative of the preferred embodiments.Accordingly, I do not intend that the scope of my exclusive rights andprivileges in the invention be limited to details of the embodimentsdescribed. I do intend that equivalents, adaptations and modificationsof the invention reasonably inferable from the description containedherein be included within the scope of the invention as defined by theappended claims.

SUMMARY OF THE INVENTION

[0022] Briefly stated, the present invention comprehends a newsilicate-based optical glass composition which has physical propertiesthat make the glass particularly well suited to use as a substrate onwhich thin film coatings may be applied to form optical interferencefilters having a narrow wavelength transmission band which is preciselycentered on a selected wavelength, in spite of variations of ambienttemperature. Stable narrow band optical interference transmissionfilters of this type are required for certain optical communicationapplications such as Dense Wave Division Multiplexing (DWDM). Accordingto one aspect of the present invention, a silicate glass composition isprovided which has a relatively large coefficient of thermal expansion,found necessary to maintain wavelength stability in optical interferencefilters fabricated by applying thin dielectric coatings onto a glasssubstrate. According to the invention, relative proportions ofcomponents of the novel glass composition may be varied to vary theresultant coefficient of thermal expansion (CTE) over an approximaterange of 91 to 120×10⁻⁷/° C. and over a preferred range of approximately105 to 120 10⁻⁷/° C. According to another aspect of the invention, asilicate glass composition for optical interference filter substrates isprovided which has a high CTE, in the approximate range of 91 to 12110⁻⁷/° C., and also has a high rigidity, i.e., has a Young's modulus ofover 85 Gpa. A high Young's modulus is desirable for glass substratesused for optical interference filters of the type used in DWDM, sincesuch filters are made using very small, thin plates having dimensions ofthe order of 2 mm×2 mm×2 mm or smaller, and are therefore subject todeformation sufficient to cause shifts in the optical properties of thefilter in response to small mechanical stresses, if the Young's modulusof the glass is not sufficiently high.

[0023] According to another aspect of the present invention, a novelsilicate glass composition is provided which has a high CTE which may bevaried in a predictable manner by changing relative proportions ofcomponents of the composition, a high Young's modulus and a relativelyhigh optical transmittance, e.g., a transmittance of 90% or greater fora sample thickness of 8 mm over the approximate near infrared (NIR)wavelength range of 1300 nm to 1600 nm, while also being resistance tosurface damage when exposed to high temperatures and high humidityconditions for prolonged time periods.

[0024] The high thermal expansion coefficient range stated above asbeing a requirement for glass used as a substrate for DWDM opticalinterference filters is higher than that of most current opticalglasses. Typically, such higher thermal coefficients of expansion areachieved only in softer, weaker glasses. But softer, weaker glasses havea lower Young's modulus, and are therefore insufficiently rigid for DWDMapplications, and also are subject to surface degradation in highhumidity, high temperature environments. Accordingly, a delicatemanipulation of glass composition is required to create a glass whichhas the properties specified above.

[0025] It has been discovered by the present inventor that requirementsof glass properties for DWDM filter substrates set forth above can befulfilled with glasses having the following composition (in mole %):

[0026] 40-62% SiO₂

[0027] 2-20% (Al2O3, B203, La203)

[0028] 8-36% alkaline oxides

[0029] 0-40% alkaline earth oxides

[0030] 0-20% of any non-color generating heavier metal oxides.

[0031] A preferred glass composition found by the present inventor to becapable of satisfying the aforementioned requirements has the followingcomposition, in mole %:

[0032] 43.3% SiO2

[0033] 7.0% Al2O3

[0034] 12.7% CaO

[0035] 7.3% SrO

[0036] 7.8% Li2O

[0037] 13.2% Na2O

[0038] 8.0% K2O

[0039] 0.7% ZrO2

[0040] 0.1% Sb2O3.

[0041] The preferred composition glass had a coefficient of thermalexpansion (CTE) of 112×10- ⁷/C over the temperature range −30° C. to+70° C., a Young's modulus E of 88.3 Gpa, and an optical transmittanceof 90.9% at 1550 nm for an 8 mm thick sample.

DETAILED DESCRIPTION OF THE INVENTION

[0042] As a result of extensive investigations on the relationshipbetween types and proportions of various glass components and theirrespective influence on the glass properties, the present inventor hasdiscovered a novel glass composition, described in greater detailhereinafter, which satisfies the aforementioned property requirements.

[0043] In the compositional investigations, each experimental melt wasmade with 200 to 250 grams of glass. The thoroughly mixed batch materialwas melted in a platinum crucible for about 90 minutes followed by a5-10 minute mechanical stirring and then a refining period of about 20minutes. Melting temperature ranged from 1350° C. to 1460° C. dependingon the chemical composition and batch materials.

[0044] Variation of each component of the aforesaid composition affectscertain glass characteristics. The effect and range of proportions ofeach component have been investigated and are explained in the followingparagraphs.

[0045] Tests performed indicated that the glass thermal expansiondepends on the type and concentration of each component in the glass.Higher concentration of a matrix component, SiO2, causes difficulty inthe glass achieving a target range of its coefficient of thermalexpansion. Lower concentrations of SiO2 makes it easier to reach targetvalues of the thermal expansion coefficient requirement, yet the glasstends to become less stable and poor in environmental resistance whenthe concentration is too low. These effects can be demonstrated by theexamples in the following tables.

[0046] In a first series of tests, the results of which are summarizedin Table 1, the concentration of SiO2 was varied from 37.4% to 65.0%. Asindicated by the test data of Table 1, if the concentration of SiO2 istoo low, e.g., 37.4%, slight devitrification of the glass occurs,resulting in decreased transmissibility. As is also shown in Table 1,increasing the concentration of SiO2 from 40% to 65% decreases thecoefficient of thermal expansion (CTE) from 126×10⁻⁷ to 91×10⁻⁷/° C.Based upon the results of test series 1, it was concluded that apreferred concentration range of SiO2 is between about 40% and about58%. TABLE 1 Composition in Mole % CTE, SiO2 R2O3 RO R2O ZrO2 Refiners10⁻⁷/° C. Glass Quality 37.4 6.9 20.0 35.0 0.7 0.1 NA Slightlydevitrified 40.0 3.6 22.6 33.6 0.0 0.1 125 Clear Glass 50.0 3.0 18.928.0 0.0 0.1 112 Clear Glass 65.0 2.1 13.2 19.6 0.0 0.1  91 Clear Glass

[0047] In a second series of tests, the results of which are summarizedin Tables 2A and 2B, it was determined that components of alkaline metaloxides in glass according to the present invention have a major effecton the CTE. Thus, as shown in Table 2A, CTE values of 106, 113 and 120were obtained for total alkaline metal oxide concentrations of 24%, 28%and 32% respectively. Similarly, as shown in Table 2B, CTE values of104, 111, and 121 were obtained for total alkaline metal oxideconcentrations of 23.1%, 27% and 30.9%, respectively. The presentinventor has also found that distribution, i.e., relative proportions ofdifferent alkaline oxides, also affects the properties of glassaccording to the present invention. Of the three most important suchoxides, Li2O, Na2O and K2O, K2O appears to have the greatest elect onincreasing the coefficient of thermal expansion (CTE). In general, itwas found that the effect on the thermal expansion coefficient followsthe order of K2O>NA2O>Li2O. On the other hand, the effect of alkalineoxides on Young's modulus follows the order Li2O>Na2O>K2O.

[0048] Based upon tests summarized in Tables 2A and 2B, it has beendetermined that a preferred total alkali content in glasses according tothe present invention lies in the range between about 15 mole % andabout 30 mole %. The exact total alkaline oxide molar concentration, anddistribution percentage between the various alkaline oxides, depend onthe glass user's required specifications; especially the CTE and Young'smodulus. TABLE 2A Composition in mole % SiO2 AL2O3 La2O3 CaO SrO BaOLi2O Na2O K2O Refiners CTE 54.9 1.6 1.6 11.8 3.0 3.0 8.1 12.0 3.9 0.2106 52.0 1.5 1.5 11.2 2.8 2.8 9.5 14.0 4.5 0.2 113 49.1 1.4 1.4 10.6 2.62.6 10.9 16.0 5.1 0.1 120

[0049] TABLE 2B Composition in mole % SiO2 AL2O3 La2O3 CaO SrO BaO Li2ONa2O K2O Refiners CTE 58.0 3.3 7.2 8.3 11.1 7.7 4.3 0.2 104 55.0 3.1 6.87.9 13.0 9.0 5.0 0.2 111 52.0 3.0 6.4 7.5 14.9 10.3 5.7 0.2 121

[0050] Test series number 3 performed in the course of development ofglasses according to the present invention revealed that percentageconcentrations of alkaline earth oxides in the glass composition haveeffects on glass properties similar to, but not as pronounced as, thoseof alkaline oxides. Thus, as shown in Table 3, a total alkaline earthoxide concentration of 10.8% comprised of 5.0% SrO and 5.8% BaO resultedin a CTE of 109, a total alkaline earth oxide concentration of 14.7%comprised of 6.8% SrO and 7.9% BaO resulted in a CTE of 111, and a totalalkaline earth oxide concentration of 18.8% comprised of 8.7% SrO and10.1% BaO resulted in a CTE of 113. TABLE 3 Composition in mole % SiO2AL2O3 La2O3 CaO SrO BaO Li2O Na2O K2O Refiners CTE 57.5 0 3.3 0 5.0 5.813.6 9.4 5.2 0.2 109 55.0 0 3.1 0 6.8 7.9 13.0 9.0 5.0 0.2 111 52.4 03.0 0 8.7 10.1 12.4 8.6 4.8 0.2 113

[0051] Test series numbers 4 and 5 were performed to determine theeffects of trivalent components Al2O3, B2O3 and La2O3 on glassesaccording to the present invention. As shown in Tables 4 and 5,increasing the concentration of B2O3 from 0 to 2.9% had the effect oflowering CTE. Since the reduction of CTE due to the inclusion of B2O3 isnot very great, and since B2O3 is known to improve glass stability andchemical resistance, and may increase Young's modulus, its presence inglasses according to the present invention is permissible. Preferably,the concentration of B2O3 will be less than 10%. TABLE 4 Composition inmole % SiO2 B2O3 AL2O3 La2O3 MgO CaO BaO ZnO Na2O K2O Refiners CTE 56.32.9 1.0 4.4 2.9 8.4 2.9 2.6 15.6 2.8 0.2 97 58.0 0.0 1.0 4.5 3.0 8.6 3.02.7 16.1 2.9 0.2 104

[0052] TABLE 5 Composition in mole % SiO2 B2O3 AL2O3 La2O3 CaO BaO ZnONa2O K2O Refiners CTE 56.2 2.9 1.0 7.8 8.3 2.9 2.6 15.4 2.8 0.2 100 57.90.0 1.0 8.0 8.5 3.0 2.7 15.9 2.8 0.2 103

[0053] In test series number 6, it was found that aluminum oxide inglasses according to the present invention tend to reduce CTE andYoung's modulus, as indicated in Table 6. However, presence of thistrivalent oxide in glasses according to the present invention isbelieved to improve glass stability and environmental resistance, andtherefore may be included in the formulation. TABLE 6 Composition inmole % SiO2 AL2O3 MgO CaO SrO BaO Li2O Na2O K2O Refiners E CTE 50.8 1.53.0 8.4 4.9 2.9 9.7 14.2 4.6 0.2 88.2 120 50.0 3.0 3.0 8.3 4.8 2.8 9.514.0 4.5 0.2 86.2 113 48.7 5.5 2.9 8.0 4.7 2.7 9.3 13.6 4.4 0.2 84.9 10647.7 7.5 2.9 7.9 4.6 2.7 9.1 13.4 4.3 0.2 NA 104

[0054] Test series number 7 was performed to ascertain the effects ofaluminum oxide and lanthanum oxide on the CTE and Young's modulus ofglasses according to the present invention. As indicated in Table 7A,increasing the percentage of La203 from 0%, 1.6%, and 3.0% whilesimultaneously deceasing the percentage to Al2O3 over the range 3.0%,1.4% and 0% caused CTE to increase to 113, 115, 116, respectively. Also,as shown in Table 7B, reducing the percentage of LA2O3 from 3.0% to 0%,while increasing the concentration of AL203 from 0.0% to 3.0%, whilemaintaining the concentration of the other components constant caused adecrease in CTE of only 116×10⁻⁷/° C. to 113×10⁻⁷/° C., and a decreasein Young's modulus E of only 90.0 Gpa to 86.2 Gpa.

[0055] Thus, the advantage of lanthanum oxide over aluminum oxide inincreasing CTE and Young's modulus is not very significant and thelatter material is cheaper, aluminum oxide may be used in place oflanthanum oxide in glasses according to the present invention. For thepresent invention, the preferred concentration for aluminum oxide isbetween about 0 to 10 mole % and that of lanthanum oxide is betweenabout 0 to 9 mole %.

[0056] It is usually possible to include a small amount of P2O5 insilicate glass without any major ill effects. However, the tolerance forsuch inclusion without causing serious detriment is quite small. We didsome experimental melts with small amounts of P2O5 and found a verylittle change in Young's modulus and thermal expansion. The magnitude ofchanges are very close to our technical measurement error limits.Therefore, it might not be of any proven effect. For the presentinvention, the limit for this component is less than 5%, its preferredconcentration is less than 2%.

[0057] For an experienced class chemist, it is possible to add othercomponents into the preferred compositions of this invention with littlevariation of the glass properties. Such components include, but are notlimited to: the oxides of tantalum, yttrium, ytterbium, zinc, zirconium,titanium, niobium, tungsten, cerium, tin and hafnium. The limit for eachof these component is believed to be approximately 0 to 8 mole %. TABLE7A Composition in mole % SiO2 AL2O3 La2O3 MgO CaO SrO BaO Li2O Na2O K2ORefiners E CTE 50.0 3.0 0.0 0 11.2 4.8 2.8 9.5 14.0 4.5 0.2 86.0 11350.0 1.4 1.6 0 11.2 4.8 2.8 9.5 14.0 4.5 0.2 87.2 115 50.0 0.0 3.0 011.2 4.8 2.8 9.5 14.0 4.5 0.2 88.6 116

[0058] TABLE 7B Composition in mole % SiO2 AL2O3 La2O3 MgO CaO SrO BaOLi2O Na2O K2O Refiners E CTE 50.0 3.0 0.0 3 8.3 4.8 2.8 9.5 14.0 4.5 0.286.2 113 50.0 0.0 3.0 3 8.3 4.8 2.8 9.5 14.0 4.5 0.2 90.0 116

What is claimed is:
 1. Glass for use as a substrate for opticalinterference filters comprising in mole percentages the followingingredients; a. 36% to 66% SiO2, b. 0 to 12% AL2O3, 0 to 6% B2O3, 0 to8% La2O3, C. 0 to 10% MgO, 0 to 16% CaO, 0 to 16% SrO, 0 to 16% BaO, 0to 8% ZnO, d. 0 to 32% Li2O, 0 to 25% Na2O, 0 to 25% K2O, 0 to 20% Cs2O,e. 0 to 8% each of Sc2O3, Y2O3, TiO2, ZrO2, HfO2, TA2O5, Nb2O5, WO2,SnO2, Ce2O3, and f. 0 to 6% P2O5, 0 to 5% PbO, 0 to 5% As2O3, 0 to 5%Sb2O3.
 2. The glass as defined in claim 1 wherein the total molepercentage of alkaline oxides lies in the approximate range from about8% to about 42%.
 3. The glass as defined in claim 1 wherein the totalmole percentage of alkaline earth oxides lies in the approximate rangefrom about 0% to about 46%.
 4. The glass as defined in claim 1 whereinthe total mole percentage of ingredients selected from the group ofAL2O3, B2O3, and La2O3 lies in the approximate range from about 0% toabout 20%.
 5. The glass as defined in claim 1 wherein the total molepercentage of heavier metal oxides including PbO, Sc2O3, Y2O3, TiO2,ZrO2, HfO2, TA2O5, Nb2O5, WO2, SnO2, and Ce2O3 lies in the approximaterange from about 0% to about 20%.
 6. The glass as defined in claim 1further defined as having a coefficient of thermal expansion lying inthe approximate range between about 91×10⁻⁷/° C. and about 120×10⁻⁷/° C.7. The glass as defined in claim 1 further defined as having a Young'smodulus of greater than 80 Gpa.
 8. The glass as defined in claim 1further defined as having an optical transmission at 1550 nm of greaterthan about 90% for a plate thickness of about 8 mm.
 9. Silicate glassfor use as a substrate for optical interference filters comprising inmole percentages the following ingredients; a. SiO2: approximately 40%to about 62%, b. (Al2O3, B2O3, La2O3) approximately 2% to about 20%total, c. Alkaline oxides: Approximately 8% to about 36%, d. Alkalineearth oxides: approximately 0% to approximately 40%, e. Non-colorgenerating heavier metal oxides: approximately 0% to approximately 20%.10. The glass as defined in claim 9 further defined as having acoefficient of thermal expansion lying in the approximate range betweenabout 91×10⁻⁷/° C. and about 120×10⁻⁷/° C.
 11. The glass as defined inclaim 9 further defined as having a Young's modulus of greater than 80Gpa.
 12. The glass as defined in claim 9 further defined as having anoptical transmission at 1550 nm of greater than about 90% for a platethickness of about 8 mm.
 13. Silicate glass for use as a substrate foroptical interference filters comprising in mole percentages thefollowing ingredients: a. SiO2: approximately 43.2%, b. Al2O3:approximately 7.0%, c. CaO: approximately 12.7%, d. Sro2: approximately7.3%, e. Li2O: approximately 7.8%, f. Na2O: approximately 13.2%, g. K2O:approximately 8.0%, h. ZnO: approximately 0.7%, i. Sb2O3, As2O3approximately 0.1% total.
 14. The glass as defined in claim 13 furtherdefined as having a coefficient of thermal expansion lying in theapproximate range between about 91×10⁻⁷/° C. and about 120×10⁻⁷/° C. 15.The glass as defined in claim 13 further defined as having a Young'smodulus of greater than 80 Gpa.
 16. The glass as defined in claim 13further defined as having an optical transmission at 1550 nm of greaterthan about 90% for a plate thickness of about 8 mm.